H2O (g) – Water Vapor

Did you know that water vapor, not carbon dioxide or methane or any of the other 4 greenhouse gases, is the biggest contributor to global warming?

If you did, nice work! If not, don’t feel too bad, because it seems that a lot of people don’t know this information. All we tend to hear about is CO2 (carbon dioxide), followed by CH4 (methane), and N2O (nitrous oxide).

Huh?

Greenhouse gases let shortwave solar radiation into our atmosphere, while keeping longwave radiation (what is bounced back from the Earth) in. While some of this process is important for keeping us warm and comfortable, too high of a concentration fo greenhouse gases throw off the balance and contribute to ‘global warming’.

So, what’s the deal with the water vapor?

Going back to basic science – the warmer the air, the more water content it can hold (think tropical areas that are deathly humid and how you tend to feel humidity more acutely in summer months). Water vapor is created when water from the Earth’s surface evaporates – something that happens naturally and is not necessarily due to human actions.

Water vapor in the air accounts for 60 to 70 percent of the greenhouse effect – so all the water molecules up there are not allowing the longwave radiation to escape – causing a warming effect. However, water vapor is removed from the atmosphere in the form of rain in about a week after entering the atmosphere, while CO2 – which accounts for about 25% of the greenhouse effect – remains up there for 200 to 300 years on average.

The more CO2 and other greenhouse gases that are emitted into the atmosphere (some are natural, some are caused by us), the warmer the air becomes, as these gases also cause a warming effect. What happens when the air is warmer? Evaporation increases, allowing for more water vapor to be in the atmosphere, thereby amplifying the warming effect. It’s a vicious cycle.

Bottom line: While water vapor is the main offender – anthropogenic related emissions like carbon dioxide add to and amplify the warming effect that water vapor has on the climate.